Signal wave transmission system



June 26, 1945. l., G. ABRAHAM SIGNAL.l wAvE TRANSMISSION SYSTEM Filedmay 12, 194s ALZQRNEV Patented `Func 26, i945 UNirao STATES PATENToFFIcE 2,379,055 SIGNAL WAVE TRANSMISSION SYSTEM Leonard G. Abraham,MadisonN. J., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., `a corporation of New York "biiutoii stay V12, 1943, Serial'No1l` 486,688" C'lims. (C1. rvs-44) This invention relates to signalwavetran n'n's'- sion systems and particularly. to systems fortransmitting a Wide band of signal' 'frequericies.

An object ofthe invention is toA transmitefliciently and economically asignalwave compri..- ing a Wide band of frequencies. f y

A related object is totransmit eiiic'iently a" signal Wave comprising aWidef bandof frequencies, utilizing available transmission linesinherently capablel of transmitting feflicientlyorilygan'arrowerfrequency band. t f i' Another object is to cheapen the'costpf' termi'- nallilte'ring equipment in ajwidej frequencylbandcarrier wave signaling systeriwithoutiapprecif ably reducing the qualityof transmission'i These objects are attained in'accordane with theinvention by a particular method fo dividing,r the signal Wave of widefrequency, ,1113, r

a complex carrier wave, into two or morparts tions each containing thewhole band of frequenf cies; a narrower frequency subband is selectedfrom one energy portion of 'the'wave'al parto'f thev energy of theselected subband `is used to balance out corresponding frequencies theother energy portion of the-Wave; the' remaining frequencies in thelatter energy portion of the signal Wave, and the selected frequencysubband Yin the rst energy portion are transmitted loverseparateavailable transmission circuits?" of' limited transmission frequencyrange 'to a receiving point, utilizing frequency shifting operations, ifnecessary, to bring them Within the frequency limits of the availablecircuits before transmission and to restore them to their riginalfrequencies after transmission; and at the receiving point the two`fredencysubbands are combined to reproduce the original signal tvavetIn another embodiment, a similar processfis employed at the transmittingend ofi@ Single Sid@- band carrier signaling system for efficientlyseparating the side-band component ,toibe transmitted from the otherside-band Ycornpnnent .and the carrier, utilizing relativelycheapflltersfw/ YThe various objects rvand features -of the inventionWill be better understoodV from the following detailed descriptionwhenread vconjunction With the accompanying draining in which are shown inblockl schematic form, different modifications of the invention appliedtoI program transmission systems. Fig. ,1,Li1lustratesthel applicationofpthe method of the4 invention to, the transmission c f,a A programwith,freguencies ranging from, sayfpfto 15,009, cycles, between tv/o,widely separated points` A v,and B, 2Where the ,only transmission mediaavailable between those points."arela4` pluralitypf tolltransmissioncircuits vor lines` ,',IL1f, TLzfe'ach 'capable oftransmitting efliciently only a. portion, say 30 toj `8,509 cycles, ofthat' frequency ranger@ v, Atgstation A, i nthe system of Eig.Ltheprogram Wave comprisingfrequencies ranging 'from about 30,eycle s tg15,000` cycles receivedeve the line from a `source (not -shown), ufhichmay be a microphone or a .telephene .line, lis fed Ain multipleinto'two branch" transmission circuits TG1 and TCz which may includeone-Way amp ifying. vrnearls A.1, .A2 for providing ai'. desired amountoffamplication in theequ'al energy portions of the. wie@ lfrequenv band..ware transf mitted over the respective branch circuits.4 In

the' upper branch circuit'TCiisa bandfpassiilter Fi which is `adaptedtofpass*a'relatively'narrcvv frequency. Subbanda te y13.',000 Cycles.Qfgthe impressed wave; (this filter u1d bea1owpas,s lte ,hai/ing, a{Gut-Oatfoo Cycles.) and .tb then c utfoff in aY reasonable frequencyinterval'to very highf loss at, say 8,500 cycles, A portion V of theenergy, f, the Selected" 30509 Y,84100 yls' Sub'an'd is transmitted outover the available 4(30 to,V 59() cyclegf-toll line vT L1 tothereceiving 'station B Where ,it is transmitted over the lreceivingcircuit RC1` through the one-way amplifying 'deviceAr to the circuit orline L2 leading to signal repro'- ducingapparatus. Another energyportionof the' selected :30 to 8,000 cycle frequency "subband 4in theoutput of lter F1 at station A passes into the @fle-Way amplifying,device ,All

,Injthe othr'brenh C twit' TCwPStatQl A, thev entire 30 to l15,00()cycle bandV of theimpressed program wave is transmitted through vthetransmission loss device C1, Which "may be f a inputpf the by-passcircuit BCi including the` phase corrector if tests indicate that phasec rrection is necessary. The 'jamplified'O t`o 8 ,pl l0 cycle waveoutput of the by'fpass 'circuit BCi is afpvparts are combined at thegram frequencies -between 8,000 and 8,500 cycles and the completefrequency band between 8,500 and 15,000 cycles.

The resulting wave comprising frequencies ranging from 8,000 to 15,000cycles, in the lower transmission branch TG2 then passes to themodulator M1 in which they are combined with a carrier wave of 7.9kilocycles supplied from the carrier source S1, to produce signalside-bands of frequencies ranging from 0.1 kilocycle to '7 .1 kilolcycles and from 15.9 kilocycles to 22.8 kilocycles, respectively. Acheap band-pass filter F2 in the output of the modulator M1, adapted forpassing frequencies up to 7.1 kilocycles and cutting off at 7.9kilocycles and higher frequencies, selects j filter cutting off above 8kilocycles with very high loss at 16 kilocycles and above, which wouldbe cheaper than the band-pass filter. Also with the latter modification,the frequencies just above 8,000 cycles in the original program wavewould still be near 8,000 cycles on the line Th2 and would be delayed intransmission over that line about the same amount as the nearbyfrequencies in the other line TL1. This might aid in reproducing theoriginal wave when the two frequency receiving end of the system.

At the receiving station B, the received' frequency subband, '7,100 to100 cycles, is combined in the modulator M2 with the carrier wave of 7.9kilocycles from the associated source S2, to step it up to the originalfrequency position, 8,000 to 15,000 cycles, which it had at the input ofthe modulator M1 in the transmitting station A. This frequency band,8,000 to 15,000 cycles, is selected by the cheap band-pass filter F3 inthe output of the modulator M2. The selected subband is then transmittedover the receiving circuit branch RC2 through the one-way amplifier A5to the circuit L2 in which by proper poling it may be made to combinewith the amplified to 8,000 cycle subband received from the receivingcircuit branch RC1, to reproduce the original 30 to 15,000 cycle bandprogram wave l The phase corrector C1 in the transmitting branch TCz atthe station A, should preferably be so designed that, with a turn-overof the frequencies, the band from 30 to about 8,000 cycles in its outputwill be almost exactly out of phase with the corresponding frequencyoutput of the by-pass circuit BC1 when they are combined at thetransmitting station A. vIf this is not accomplished, there will befrequencies below 8 kilocycles at the input of the first modulator M1,and another filter will be required there for cutting off thosefrequencies. For the alternative scheme referred to above, employing a16-kilocycle carrier wave for the modulator'M1 and a low pass filtercutting off above 8 kilocycles for the filter F2 in the output of themodulator, this might not be necessary. In either case, a very cheapfilter could be used if complete cancellation could be obtained, say fora few hundred or a thousand cycles below 8 kilocycles. In the regionfrom 8 to 8.5 kilocycles, phase correction for the filter F1 in theupper transmitting branch TG1 would be of advantage, but is perhaps notessential. For example, suppose that at 8.2 kilocycles, the wave in theoutput of the by-pass circuit BC1 and the wave in the output of thephase corrector C1 in the lower branch circuit TG2 would be exactly inphase, instead of out of phase, at the point of combination. With a lossfor the filter F1 in the upper branch circuit TG1 of, say, 10 decibels,the level of the currents in the lower branch circuit TC2 should bereduced by about 3.3 decibels by the combination, but with degrees phaseerror would be increased by 2.4 decibels. With no line distortion,however, there would be no final error at the receiving end of the lineso that the only effect would be to slightly change the power on theline. Of course, delays which are great enough to cause echo effectscould not be tolerated.

It is apparent, of course, that a system based on the 'same principlesdescribed above in connection with Fig. 1, could be devised to transmita band which is wider than 30 to 15,000 cycles, or to transmit afrequency band by dividing it into more than two parts. For example, letus sup pose it is desired to transmit a 30 to 22,000 cycle band ofsignal frequencies. VThe upper branch circuit TCi with the 30 to 8,000cycle band-pass filter F1, and the by-pass circuit BC1, could be used asshown, and the wave in the lower trans mitting circuit TC2 resultingfrom the combination of the 30 to 22,000 cycle band in the output of thephase corrector C1y with the wave output of the by-pass circuit BC,would have frequencies ranging from 8,000 to 22,000 cycles. This wavethen could be split into two frequency parts by transmitting it inmultiple into two auxiliary band circuits, similar to TC1 and TCz, onewith an 8,000 to 15,000 cycle band-pass (or 15 kilocycle low-pass)filter. A portion of the energy of the frequency subband in the outputof that filter could be used in the same way as described for thearrangement of Fig. 1 to cancel out correspending frequencies in thewave in the other auxiliary branch. After shifting all three frequencysubbands thus obtained to some desired place in the frequency spectrumdetermined by the frequency limits of three'transmission lines availableand transmitting them so shifted over those lines to the receivingstation, they can be there shifted back to their original frequencyvalues and be combined in reverse order to obtain the original signalfrequency band of 30 to 22,000 cycles. Another possibilbity would be toproduce an 8 kilocycle program circuit from two 4 kilocycle messagecircuits by similar means.

It is also apparent that different levels and predistortion could beused for the two or more branches on lthe transmission line, since theenergy content will be quite different. Compandors could be used on someor all of the bands separately. Some rearrangements of the filter'cutoffs andthe selection of different carrier frequencies might bedesirable in order to have the two branches transmit more nearly thesame frequency bands.

The method which has been described above in connection with Fig. 1, issuperior to that of using two filters for selecting the frequencies tobe transmitted over the two lines, one transmitting 0 to 8 kilocyclesand the other 8 to 15 kilocycles, -because of the lower cost of thesecond (lower) branch than an 8 to 15 kilocycle filter, and because therequirements on the correspondence around 8 kilocycles of the twofilters in the latter scheme to give perfect transmission of the totalfrequency band, are extremely difficult and perhaps impossible toobtain. No such requirement is necessary with the scheme illustrated inFig. 1 or the modifications thereof described above.

A second useful application of the method of the invention isillustrated in Fig. 2 which shows schematically a high frequency carrierwave system utilized for transmitting a program wave of frequenciesranging from 30 to 15,000 cycles, from station A to station B.

As indicated, at station A in Fig. 2 the program band of 30 to 15,000cycles is combined in a modulator M3 with a carrier wave of 80kilocycles from an associated source S3, to produce modulation productsof frequencies ranging from 65 to 95 kilocycles. lt is desired totransmit only the upper side-band, 80 to 95 kilocycles, of themodulation products. It is technically difficult to make a band-passfilter passing 80 to 95 kilocycles with sufficiently sharp cut-off toeliminate the carrier and lower side-band. On the other hand, an 80 to88 kilocycle band-pass filter and an 88 kilocycle high-pass filter or an88 to 95 kilocycle band-pass filter may be produced easily, particularlyof a few hundred or a thousand cycles for cutting off below 88kilocycles are unimportant.

In the arrangement of Fig. 2, the energy of the modulation products offrequencies ranging from 65 to 95 kilocycles in the output of themodulator Ma is divided between the upper transmission branch TCa andthe lower transmission branch TC4. The wave in the upper branch TCS istransmitted through a band-pass filter F4 adapted to pass the frequencysubband, 80 to 88 kilocycles, and to cut off with great loss at, say88.5 kilocycles. The other energy portion of the complex carrier wave inthe output of the modulator M3 passes through a phase corrector C2 inthe lower circuit branch TC4, having a function similar to that of thephase corrector C1 in the system of Fig. 1. The 65 to 95 kilocycle wave`in the output of the phase corrector C2 is combined with the amplied 80to 88 kilocycle wave in the output of the by-pass circuit BCz includingthe one-way amplifier Ae, the input of which is fed from the uppertransmission branch TG3 in the output of the band-pass filter F4, insuch phase and magnitude as to eliminate from the 65 to 95 kilocyclewave transmitted over the branch TC4 the frequencies below 88kilocycles. The remaining frequencies 88 to 95 kilocycles of the wave inthe lower branch TC4 are transmitted through the high-pass filter F5having a cut-off at 88 kilocycles, which eliminates the lower sideband,80 to 88 kilocycles, and the carrier component of 80 kilocycles. Theremaining frequencies 88 to 95 kilocycles in the output of the filter F5pass through the one-way amplifier Aa to the outgoing line La in whichthey are combined with the other energy portion of the 80 to 88kilocycle wave selected by the band-pass lter F4 in the upper branch TG3and transmitted over the output of that branch through the one-wayamplifier A1 to the line L3. Thus, the wave transmitted over the line Lato station B comprises a single side-band with frequencies ranging from80 to 95 kilocycles, approximately, the carrier component and the otherside-band having been suppressed in the manner just described.

It may be pointed out that when the 88 kilocycle high-pass lter is usedfor the filter F5 in the lower branch T04, an equivalent effect could beobtained by using only an kilocycle highpass filter for the filter F4 inthe upper branch TG3 connected to the output of modulator M3. The use ofan 88 to 95 kilocycle band-pass filter in place of the high-pass filterF5 is a preferred arrangement.

Since the filtering and balancing circuits above described are alllocated at the input terminal of the line La in the system of Fig. 2,phase or amplitude distortion in that line does not affect the output,and phase corrector C2 may not be necessary except to avoid echoeffects. At the receiving station B in the, system of Fig. 2, thereceived side-band, 80y to 95 kllocycles, is combined with an 80kilocycle carrier wave from the source S4 in the modulator M4 to providein the output of that modulator, side-bands of frequencies ranging from0 to 15 kilocycles and from to 165 kilocycles, and a simple low-passfilter F6 in the output of the modulator, cutting off somewhat above 15kilocycles could be used to suppress the carrier and upper side-band toreproduce the 30 to 15,000 cycle program wave applied to the input ofthe system.

In the case of Fig, 2, also, more than two divisions of the band in theoutput of the modulator M3 could be used, either for a wider band or topermit cheaper filters than the 8 kilocycle filters.

It is obvious that the methods described in connection with Figs. l and2 offer certain practical advantages. For example, the methods wouldhave 'considerable economical advantage in the case where most programcircuits are to have an 8 kilocycle'band and the 15 kilocycle band isonly rarely required.

Various modifications of the circuits illustrated and described whichare within the spirit and scope of the invention will occur to personsskilled in the art.

,What is claimed is:

1. The method of transmission which consists lin selecting from oneenergy portion of a wave comprising a wide band of frequencies, arelatively narrow frequency subband, utilizing one energy part of theselected subband to effectively balance out the correspondingfrequencies in another energy portion of said wave, selecting a secondfrequency subband from the remaining frequencies in said other energyportion of said wave, transmitting said second frequency subband andanother energy part of the first selected subband over separate paths toavreceiving point and combining the received subbands thereat to producea combination wave containing all of their frequencies.

2. In a signal transmission system comprising a plurality of signaltransmission lines of limited frequency transmission range extendingbetween geographically separated stations, the method of transmittingefficiently between said stations a signal wave comprising a wide rangeof frequencies, which consists in dividing said wave at one of saidstations into two energy portions each comprising said wide range offrequencies, selecting from one of said energy portions a frequencysubband within the frequency range of one of said lines, transmittingone energy part of the selected subband over said one line to the otherstation, utilizing another energy part of said selected subband at saidone station to effectively balance out corresponding frequencies in theother energy portion of said wave, shifting the frequencies remaining insaid other energy portion of said wave to a position in the frequencyspectrum within the frequency transmission range of anothery of saidlines and transmitting the frequency band in its shifted positionthereover to said other station, shifting the frequency band receivedover said other line back to its original position in the frequencyspectrum, and combining at said other station the latter frequency bandso shifted with the frequency subband received over said one line insuch manner as to reproduce the original signal wave.

3. The method of separating one side-band component from the otherside-band component and the carrier component in a complex carrier wave,which consists in dividing said complex i, wave into two energy portionseach containing all its component frequencies, selecting from one ofsaid energy portions a frequency subband comprising a portion of thefrequencies in said one side-band component, combining one energy partof the selected frequency subband with the other energy portion of saidcomplex wave in such phase and magnitude as to effectively balance outthe corresponding frequencies in said other energy portion, selectingfrom the remaining frequencies in said other energy portion of said wavea second subband of frequencies corresponding to those in the otherportion of said one side-band component, and combining said secondselected frequency subband with the other energy part o f the firstselected frequency subband to produce f said one signal side-bandcomponent` 4. The method of transmitting a signal wave of a wide band ofsignal frequencies which consists in modulating said wide frequency bandsignal Wave at a transmitting point with a high frequency carrier waveto produce a complex carrier wave comprising upper and lower side-bandcomponents and a carrier component, dividing said complex carrier waveinto two energy portions each containing all of its frequencies,selecting from one of saidv energy portions a frequency subbandcomprising a portion of the frequencies within one of said side-bandcomponents, comfbining a part of the energy of the selected subband withthe other energy portion of said complex wave in such phase andmagnitude as to effectively balance out corresponding frequencies insaid other energy portion of said Wave, selecting from the remainingfrequencies in said other energy portion of said wave a second frequencysubband comprising the other portion of the frequencies in said oneside-band component, combining said second selected frequency subbandwith the other energy part of the first selected frequency subband so asto produce said one sidehand component, transmitting said one side-bandcomponent to a receiving point, modulating the received side-bandcomponent thereat with a carrier wave of the same frequency as that ofthe high frequency carrier wave utilized in the modulating step at thetransmitting point, to produce modulation products including as onesideband component the original wide band of signal frequencies andselecting that side-band component.

5. The method of transmission which consists in dividing a wave of awide band of frequencies into a plurality of energy portions, selectinga relatively narrow frequency subband from one of said energy portionsof said wave, utilizing one energy part of the selected frequencysubband to effectively balance out the corresponding frequencies inanother energy portion of said Wave, effectively transmitting at least aportion of the remaining frequencies in said other energy portion ofsaid wave and the other energy part of said selected frequency subbandover respectively different circuits to a given point and therecombining them in a common circuit to produce a combination wavecontaining all their frequencies.

LEONARD G.ABRAHAM.

